1. Field of the Invention
This invention relates to a method for managing the size of a digital word representing a signal amplitude in any digital system having finite word length processing capacity that digitally combines more than one signal into a single composite signal. More specifically, the invention relates to a power management system in a base station transceiver system which uses a digital combiner in the transmit path to combine digital signals to control combiner overflow.
2. Description of the Relevant Art
A conventional cellular phone system includes a plurality of cells, mobile units, a plurality of base transceiver stations (BTS), communication lines, a mobile telecommunications switching office (MTSO), an interface and a switched telephone network. A fixed number of channel sets are distributed among each BTS which serves a plurality of cells, arranged in a predetermined reusable pattern. The mobile unit, in a cell, communicates with the BTS, via radio frequency (RF) means.
The BTS communicates with the MTSO via communication lines. The MTSO communicates with the switched telephone network via the interface. Each BTS relays telephone signals between mobile units and a mobile telecommunications switching office (MTSO) by way of the communication lines. The communication lines between a cell site and the MTSO, are typically T1 lines. The T1 lines carry separate voice grade circuits for each radio channel equipped at the cell site, and data circuits for switching and other control functions.
An advanced BTS architecture combines a digital approach to transceiver design with broadband radio technology. This architecture uses a single broadband radio and an FFT based channelizer to digitally extract all receive channels simultaneously. An analogous technique is used on the transmit side to combine multiple channels using an inverse FFT combiner for radio transmission. Processing one composite digital signal allows substantial efficiencies compared to narrowband base stations through the reduction of components. This architecture is described fully in U.S. Pat. Nos. 5,535,240 and 5,537,435, the contents of which are hereby incorporated by reference. In short, a receiver section receives a plurality of carrier frequency communication channels and outputs digital signals representative of the contents of the plurality of communication channels. Each carrier frequency typically contains a number of channels in accordance with a TDMA format or other suitable division format. The receiver section contains an FFT-based channelizer that processes the digital signals output by a wideband digital receiver and couples respective channel outputs to a first plurality of digital signal processor units. The digital signal processor units process (e.g. demodulate) respective ones of the digital channel signals and supply processed ones of the digital channel signals at respective output ports for distribution to an attendant voice/data network.
On the transmit side, a transmit section contains a plurality of digital signal processors, respectively corresponding to a plurality of incoming (voice/data) communication signals to be transmitted over respectively different frequency channels. The processed (modulated, encoded) outputs of the DSP units are supplied to an inverse FFT digital combiner. The inverse digital FFT combiner supplies a combined multichannel signal which is then D/A converted, amplified by a high power multi-carrier power amplifier (MCPA), then supplied to a wideband transmitter which transmits a single multiple frequency communication channel signal.
In order to remain competitive in an increasingly crowded market, wireless equipment manufacturers experience constant pressure to reduce their costs. One way to reduce the overall cost of a cellular phone system is to re-design individual system components or software to operate more efficiently. For example, time dependent multiple access or TDMA divides each carrier frequency into multiple time slots. For example, if eight time slots are used, eight separate calls can be placed on each carrier frequency, multiplying a system's capacity by a factor of eight. A TDMA scheme assigns a specific time slot for each call's use during a conversation. Code division multiple access or CDMA is another transmission technology. Rather than separating frequencies by time as in TDMA, CDMA separates calls by code. In CDMA, every bit of every conversation gets tagged with a separate code.
Increased cellular system efficiency can also be realized through the use of power management techniques. A practical cellular system has limited power capabilities. It would be economically desirable to accommodate more system users while maintaining a reasonable power level per user. Such efficiencies can theoretically be realized because of the less than maximum power normally allocated to traffic channels and the random phase relationship of various channels. However, in a system that uses a digital combiner in the transmit path, such a method presents difficulties associated with possible combiner overflow. Failure to address this issue can result in compromised transmitted signal quality.
Digital combiners have a finite bit processing capability. In a TDMA system, combiners process bits on a time slot by time slot basis, combining the bit streams of all active channels sharing the same time slot. This bit processing capability is exceeded when the sum total of the digital inputs in a given time slot result in a composite output that exceeds a pre-determined combiner limit, at which point combiner overflow will occur. Digital overflow is undesirable because it can destroy the transmission signal to noise ratio, distort the transmitted signals and even possibly disable the entire BTS. One approach for controlling overflow of the digital combiner is by digitally reducing the digital representation of the input carrier amplitude input to the combiner. For example, overflow can be avoided entirely if the digital representation of carrier power levels are limited so that overflow cannot theoretically occur. This would correspond to power limitations based on a worst case calculation that assumes all active carriers are at full power and are perfectly phased.
Simply limiting the digital amplitude of all carriers input into the combiner will not provide a satisfactory solution to the foregoing combiner overflow problem since the resultant combiner digital output will often be significantly less than required to drive digital to analog conversion circuitry to full scale. Assuming the MCPA has sufficient gain, it could theoretically provide the power desired by the cellular user even if digital representations of carrier power levels output from the combiner are limited to prevent overflow. However, power amplifiers always have limited gain and are often designed to operate most efficiently near their limit for economic reasons. Second, even assuming the power amplifier can supply sufficient power to provide the mobile user with the desired power level, a reduction in digital signal amplitude output from the combiner will result in degraded signal quality because the carrier to noise level will be fixed at the output of the Analog to Digital (D/A) converter. The noise floor of the D/A is fixed and C/I is maximized by maximizing the carrier digital level into the D/A. The additional power provided by the amplifier will amplify the signal and the noise equally, maintaining the non-optimal C/I result from the D/A. Therefore, additional amplifier gain would not achieve the same benefit that maximizing the input digital word to the combiner would achieve.
Fortunately, a theoretical worst case power configuration is unlikely to occur. Some carriers may not be active. In addition, most carriers will not be at maximum power because in an actual deployment it is not expected that all carriers will be operating at or near full power. Full power for all carriers would be a statistically infrequent event because it is expected that BTS users will be randomly scattered in a standard cell and will require a different downlink power amounts. Since it is not expected that all carriers will require maximum power simultaneously, a smaller number of carriers can use relatively larger amounts of power than would be possible if all carriers were transmitting maximum power.
Finally, while it is possible that in the random course of events all carriers could be in phase with one another, this would be statistically infrequent event. Even if most carriers find themselves phased at a given instant during a given TDMA time slot, the magnitude of the buildup will be dissipated during the next burst because the carrier phase relationships will have randomly changed.
Therefore, there is a need for maximizing available carrier power and at the same time limiting actual digital overflows of the combiner. A dynamic digital overflow management system will enable more efficient usage of system power allowing a greater number of users and more power per user without risking substantial combiner overflows which can adversely affect service.